The present invention relates to speech communication and devices for the same and, more particularly, to a method and system for clarifying acoustical speech and acoustical audio in noisy environments.
Communications devices commonly are used in noisy environments. One common example of the use of such a device in such an environment is the use of a cellular telephone inside a vehicle such as an automobile. The noises in this environment include both the noise of the vehicle itself, for example, the noise of the vehicle's engine; noise, such as wind noise, resulting from the movement of the vehicle; and noise received from other vehicles.
In the case of rhythmic noise, it is known to generate a cancellation signal approximately equal in amplitude and opposite in phase to the noise. This cancellation signal is added to the noise to largely cancel the noise.
In the case of a one-way communications device, such as a car radio, the simplest way to overcome environmental noise is to simply raise the volume, i.e., increase the power of the received signal so that the received signal overwhelms the noise. A variety of methods are known for doing this automatically. Tokumo et al., in U.S. Pat. No. 4,476,571, teach an automatic sound control device for a car stereo. Ambient noise is low-pass and bandpass filtered before being used as a control signal for automatic volume control, to make sure that “noise” in the speech frequency band is ignored, so that the device does not drown out speech along with true ambient noise. Zwicker et al., in U.S. Pat. No. 4,868,881, teach the spectral decomposition of both a car radio signal and an ambient noise signal into three frequency subbands. The envelopes of the signals in the three subbands are delayed, smoothed and compared. On the basis of the comparison, each subband of the radio signal is amplified to overcome the corresponding noise subband.
These prior art methods ignore certain psychoacoustic phenomena associated with the human perception of audible signals, as described, for example, in E. Zwicker and H. Fastl, Psychoacoustics, Facts and Models, Springer, second updated edition (1999), pp. 74–84.
One such phenomenon is frequency masking. If an acoustic signal includes two similar frequency components, and the amplitude of one frequency component is significantly greater than the amplitude of the other frequency component, then the listener perceives only the stronger of the two frequency components.
Another such phenomenon is time masking. This phenomenon is illustrated in FIG. 1, which shows the amplitude waveform 12 and the amplitude envelope 14 of a short, high-frequency whistle 10 as a function of time. Also shown in FIG. 1 are a forward time masking extension 16, a backward time masking extension 18 of envelope 14, and an amplitude level 20 of ambient noise which may be white noise or which may be single frequency noise at a frequency close to the frequency of whistle 10. A listener who pays attention to whistle 10 will also perceive a subsequent whistle of similar spectral properties until forward time masking extension 16 falls below level 20 of the ambient noise, even if the amplitude envelope of the subsequent whistle is below level 20 of the ambient noise. Furthermore, the listener also perceives all of a precursor 22 of whistle 10, even though envelope 14 is below ambient noise amplitude level 20 for much of the duration of precursor 22, because backward time masking extension 18 is above ambient noise amplitude level 20 for the duration of precursor 22. Backward time masking is believed to reflect retroactive processing of the signal received by the listener, to recover precursor 22 from the ambient noise. The duration of forward time masking is on the order of about 200 milliseconds. The duration of backward time masking is on the order of about 5 to 50 milliseconds.
A third such phenomenon is wide band masking, in which a burst of signal temporarily masks ambient noise.
Many communications devices, such as hand-held cellular telephones, are small and lightweight, and therefore place a premium on low power consumption. Therefore, there is thus a widely recognized need for, and it would be highly advantageous to have, methods and associated communications devices that exploit psychoacoustic phenomena to reduce the power consumed in the boosting of an incoming signal over ambient noise.